1. Field of the Invention
This invention relates to a display apparatus and a driving controlling method, and particularly to the display apparatus and the driving controlling method that reduce power consumption of a display apparatus.
2. Description of the Related Art
A panel of the planar self-luminous type which uses an organic electroluminescence (EL) element as a light emitting element has been and is being developed energetically in recent years. The organic EL element has a diode characteristic and utilizes a phenomenon that, if an electric field is applied to an organic thin film, then the organic thin film emits light. Since the organic EL element is a self-luminous element whose power consumption is low because it is driven by an applied voltage less than or equal to 10 V and which itself emits light. Therefore, the organic EL element has a characteristic that it does not require an illuminating member and reduction in weight and thickness is easy. Further, since the response speed of the organic EL element is as high as approximately several μs, the EL panel has an advantage that an after-image upon display of a dynamic image does not appear.
Among various EL panels, a panel of the active matrix type wherein a thin film transistor (TFT) as a driving element is formed in an integrated state in each pixel is being developed energetically. An active matrix EL panel is disclosed, for example, in Japanese Patent Laid-Open Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791 and 2004-093682.
Incidentally, it is generally known that a current-voltage characteristic, that is, an I-V characteristic, of an organic EL element deteriorates as time passes, or in other words, suffers from time-dependent deterioration. In a pixel circuit wherein particularly an N-channel TFT is used as a driving transistor for current-driving an organic EL element, if the I-V characteristic of the organic EL element suffers from time-dependent deterioration, then the gate-source voltage Vgs of the driving transistor varies. Since the source electrode side of the driving transistor is connected to the organic EL element, the emitted light luminance of the organic EL element is varied by the variation of the gate-source voltage Vgs of the driving transistor.
This is described more particularly. Where the organic EL element is connected to the source electrode side of the driving transistor, the source potential of the driving transistor depends upon the operating point of the driving transistor and the organic EL element.
FIGS. 1A and 1B illustrate an operating point of a driving transistor and an organic EL element, respectively. Referring to FIGS. 1A and 1B, the axis of abscissa indicates a drain-source voltage Vds of a driving transistor, and the axis of ordinate indicates source-drain current Ids of the driving transistor.
In an initial state, the driving transistor and the organic EL element have an operating point at a position illustrated in FIG. 1A. Then, if the I-V characteristic of the organic EL element deteriorates, then since the operating point of the driving transistor and the organic EL element varies as is shown in FIG. 1B, even if the same voltage is applied to the gate electrode of the driving transistor, the source potential of the driving transistor varies. This varies the source-gate voltage Vgs of the driving transistor, and consequently, the value of the current to flow to the driving transistor varies. As a result, also the value of current flowing to the organic EL element varies, and consequently, the emitted light luminance of the organic EL element varies.
Further, particularly in a pixel circuit which uses a polycrystalline TFT, in addition to the time-dependent deterioration of the I-V characteristic of the organic EL element, a transistor characteristic of the driving transistor varies as time passes or a transistor characteristic differs among different pixels due to a dispersion in the fabrication process. In particular, individual pixels indicate a dispersion of a transistor characteristic of the driving transistor. The transistor characteristic may be a threshold voltage Vth of the driving transistor, a mobility μ of a semiconductor thin film which forms a channel of the driving transistor, and so forth. It is to be noted that such a mobility μ as described above is hereinafter referred to simply as “mobility μ of the driving transistor.”
The transistor characteristic of the driving transistor can be represented by the following expression (1):
                    Ids        =                              1            2                    ·          μ          ·                      W            L                    ·          Cox          ·                                    (                              Vgs                -                Vth                            )                        2                                              (        1        )            where Ids represents the source-drain current of the driving transistor, Vgs the gate-source voltage of the driving transistor, L the channel length, W the gate width, and Cox the gate oxide film capacitance per unit area.
If the threshold voltage Vth or the mobility μ of the driving transistor differs among different pixels, then a dispersion arises in the source-drain current Ids flowing to the driving transistor for each pixel as apparently recognized from the expression (1). As a result, even if the same voltage is applied to the gate electrode of the driving transistor among different pixels, a dispersion in emitted light luminance of the organic EL element appears among the pixels. This damages the uniformity of the screen image.
Thus, a pixel circuit has been proposed which is provided with various correction or compensation functions in order to keep the emitted light luminance of the organic EL element fixed without being influenced by time-dependent deterioration of the I-V characteristic of the organic EL element, a time-dependent variation of a transistor characteristic of the driving transistor and so forth. The pixel circuit of the type described is disclosed, for example, in Japanese Patent Laid-Open No. 2006-133542.
The correction function may be a compensation function for a characteristic variation of the organic EL element, a correction function against a variation of the threshold voltage Vth of the driving transistor, a correction function against a variation of the mobility μ of the driving transistor or a like function. In the following description, correction against a variation of the threshold voltage Vth of the driving transistor is referred to as “threshold value correction,” and correction against a variation of the mobility μ of the driving transistor is referred to as “mobility correction.”
Where each pixel circuit is provided with various correction functions in this manner, the emitted light luminance of the organic EL element can be kept fixed without being influenced by time-dependent deterioration of the I-V characteristic of the organic EL element or a time-dependent variation of a transistor characteristic of the driving transistor. As a result, the display quality of the display apparatus can be improved.